The present invention relates to load sharing apparatus for load sharing between a plurality of power supplies.
In systems which are required to operate with high reliability, it is often necessary to operate multiple power supplies in parallel, such that if one power supply should fail, a sufficient number of power supplies remain in operation for the system to continue to operate. This is known as “n+1 redundancy”.
For example, 4 power supplies rated at 300 W would be required to supply a 900 W load in a 3+1 redundant system, assuming that the power supplies share the load equally.
In practice, however, power sharing accuracies of +/−10% of full load are not uncommon, due to variations in output voltage setting accuracy and load regulation characteristics between power supplies.
Thus, in the above example, at least one additional power supply is required, to ensure that no one power supply is caused to operate above its 300 W rating.
There are two established techniques used for improving the sharing accuracy between parallel connected power supplies.
One method is to use a share bus. A share bus is an analogue or digital interface between the parallel connected power supplies, which forces the power supplies to output the same current. However, this solution is not appropriate for high reliability applications because failure of the share bus can cause the whole system to fail.
The second method is to use a technique known as droop sharing. In this technique, the load regulation of a power supply, i.e. the variation of output voltage with current, is deliberately made higher. This forces the output voltage to reduce as the power supply is loaded.
Where power supplies are parallel connected, the power supply with the highest output voltage will deliver current to the load. However, if droop sharing is applied, the output voltage of that power supply will reduce as it is loaded, thereby forcing the other supplies to deliver some current as well. The amount of droop, over the load range (i.e. the total reduction in voltage over the load range), needs to exceed the variation in output voltage setting accuracy if all supplies are to deliver some current to the load. However, if good sharing accuracy is required, the amount of droop must be substantially higher.
A known technique which allows good sharing accuracy at full load, is to apply droop only when the output current or power of each power supply exceeds a threshold load value close to 100% of full load, by applying a large dynamic impedance. In this way, the output voltage of the respective power supply falls significantly for only a small change in current above this threshold. This ensures that load sharing takes place before any one power supply exceeds is load rating.
However, this technique does not allow for load sharing at lighter loads, with the result that only one or some of the power supplies deliver power to the load during normal operation. This results in a temperature difference between the power supplies and increases the spread in their mean time to failure (MTTF).
Moreover, the above technique does not allow peak current to be drawn from the power supply, because the output voltage will become low above 100% load.